This invention relates to improved chromium-free additives for water-clays based drilling fluids in the drilling of oil, geothermal or gas wells.
For the drilling of oil or geothermal wells, at present, chromium-iron lignosulfonate, chromium humate, their combinations or the like substances are mainly used as additives for the water-clays based drilling fluids (hereinafter termed as "additives"), performing excellent advantages.
However, since the additives contain chromium, they tend to be recently restricted in use from the aspect of environmental pollution. Thus, the additives, which cause no serious pollution in the environment, are strongly desired to be developed instead of the above-mentioned high performance chromium-containing additives.
In chromium free additives, there are a mixture of sodium humate, potassium humate and natural fossil resin, and sodium and/or potassium humate and/or lignosulfonate partially complexed with zirconium compound or zirconium compound and iron compound. However, these various chromium-free additives have small stability to high temperature, and small action of dispersion and peptization. Therefore, the drilling mud containing the additives tends to increase its viscosity to gel formation by high temperature, increasing solid content or various contaminations such as sodium chloride and cements, thereby resulting in difficulties in drilling wells.